[Sn00dles]

Hey all,
I am currently brainstorming a new build and my co-worker recommended using air shocks in my setup. I know little about air shocks other than they are only for light vehicles (no problem) easily tuneable, neatly packaged, and very expensive. I have heard that they can do some weird things when you are flexed up but other than that I am very unaware of the finer points on air shocks.

Does anybody have experience with air shocks in their zook or would be able to give me more information on the finer points of air shocks, I have tried to find information online but it seems rather scarce.

Cheers

[Gwagensteve]

This is the best tech article on air shocks I've seen

http://www.billavista.com/tech/Articles ... index.html

Brand is unimportant from a tech point of view, they all work the same way.

I don't have personal experience with them, but I've been around the scene for a long time and have watched plenty of vehicles running them in competition.

I don't believe they are a feasible option for a road driven car at all and might not really be an option for a trail car.

They are hard to tune, because adjusting spring rate (nitrogen pressure) ALWAYS changes raise height. Changing temperature also changes ride height. The effective spring rate curve is nothing like a coil spring, it's sort of exponential (you'll see that in the graphs in the Billavista article) Basically, everything is interrelated - changing oil volume changes spring rate and ride height, etc etc.

Obviously, if they leak gas at all you have no springs.

Whilst they have been used in rock crawling and tuff truck locally, my understanding is that most of these guys didn't really chase the ultimate performance out of them - generally they would fill them with oil at full compression until they were full and then add nitrogen until they got the ride height they wanted. There were (is) two reasons for this - they only needed to work at low speed and the second reason is about the perceived big weakness of air shocks... "push off"

Air shocks tend to fully droop very easily - which is good for flex on an obstacle, but it's bad if you're on a side angle or climbing, where small changes in weight transfer result in very big changes in ride height. This is because air shocks have a very flat spring rate curve until they are almost fully compressed, when spring rate jumps alarmingly. Most low speed applications will have them set with 50-75mm of compression, which is just before they hit this high spring rate area, so they feel good on compression at this point - working like a "normal" spring, but that leaves 10 or more inches of droop that is very responsive to small changes in load - start climbing a hill and the rearward weight transfer will see the front come up alarmingly. Part of the reason is there's a very small amount of gas doing the springing.

The answer in competition was to run suck down winches to pull the axle towards the car and stop this happening. Practical on a comp stage at tuff truck but not generally practical on a trail.

There are large diameter, bypass air shocks available but these are very expensive, very specialised and much harder to package. I'm sure they're also a complete pain to setup. They have been used successfully in higher speed applications though - I recall Shannon Campbell was successful in an early KOH on 2.5" bypass air shocks, but has since switched to coilovers.

You haven't provided any more context about your application - use, suspension design etc so I can't provide more advice really, suffice to say you'll need well designed link suspension with the absolute minimum of geometry compromises to get the best out of air shocks.

But of trivia that goes some way to describing the tuning problem - I run 2.5" king shocks in my Sierra (it's a large/heavy sierra) with very soft springs. Spring rate is ~100lb/". I set my car up with the shocks ungassed. Spec is 150psi of nitrogen. When I gassed the shocks to spec, ride height went up 50mm. When I had my shocks valved, I enquired if, for a relatively light car, I really needed 150psi of gas, and the answer was no, I could run 80 psi, which didn't upset my ride height. Just some context on how sensitive gas pressure is - If I had to run 150psi theoretically I would have had to re fabricate my spring mounts.

[Gwagensteve]

PS, just realised I didn't really explain why setting the shocks up with minimum gas, as was done locally, was seen as desirable re push-off. The smallest volume of gas was seen as providing the fastest decline in spring rate so most of the travel would be very, very lightly sprung, the reasoning that this would prevent push off. In extreme cases though, this resulted in very little or no spring rate in the last few inches of travel, like having loose springs in a coil sprung car. This is detrimental to capability as a wheel that has no downward spring pressure on it can't provide much traction, and can also flop and bounce around which is detrimental to stability. As I discussed, suckdown winches were used to prevent this.

With coilovers, tender springs are used to keep the wheel loaded at the full extent of travel even if the "main" spring(s) have run out of travel. I use tender springs in the rear of my car to keep the wheels loaded at full droop. This isn't possible for air shocks

Do some reading on ORI struts, these use gas on both sides of the piston to attempt to provide a more linear spring rate and prevent push off. There are lots of opinions about how effective these are, and they are quite bulky and arguably more expensive than quality coil overs. There is also relatively little tech on tuning them locally as they're not popular.

If you're building a trail car that will see a mix of obstacles and general tracks, I'd have to recommend coilovers rather than air shocks.

[Sn00dles]

Thanks for all the info Steve!

After reading the article you shared it seems like a fantastic option, the harsh increase in pressure seems to be minimized by running a lighter rig as I'm planning mine to be. But just in case I will give you some more details on my build concepts and see what you think. The height tuning does seem like a unique problem that I'll need to look into though and some light sway bars could help with the flat spring rate. I'm really trying to prioritize simplicity so the packaging plus knowing that once I buy air shocks all the tuning is right there (no buying and rebuying springs) is a huge plus to me.

My plan is to build a trail rig while retaining as much of the zook look as I can, mostly for trail use but the toughest competition it will see is the On All 4s competition in Colo Heights which it would be in Outlaw class.
I've got an SJ70 chassis and some GQ diffs, planning to convert a tin top body I've got into a ute. It'll have double triangulated 4 link front and rear with hydraulic steering to keep things simple. This will be my first purely trail build so I'm trying to have as few moving pieces as possible.

It'll mostly be rock crawling in the wattagans so aside from the sun the shocks shouldn't heat up too much.

[Gwagensteve]

I just reread that article as I first read it many years ago and they didn't appeal then.
After rereading the article, they still don't really appeal. They just seem too flaky - too many competing and interrelated variables.

Whilst Billavista's article is excellent, it's important to consider a few things.

This tech is nearly 20 years old. There are many more people running coilovers now and they are well understood. Note that Billavista doesn't even mention coilovers as a point of comparison except to say they're expensive and implied they were a bit exotic.

What he does mention, however, was that he was running a very average coil and separate shock setup and there were lots of problems with his links. He was also running a home brewed radius arm setup on the front which have undesirable characteristics such as high (and sometimes variable) roll stiffness and bushing bind.

Like lots of people back then (and now, to some extent) he was running a very compromised suspension with springs working well outside their design parameters, and shocks that were not tuned to his application.

What is implied is through this process he fixed lots of other issues with his suspension - the air shocks were only one factor in this.

What is undeniable is that compared to a coilover (or even a separate spring and shock that's well chosen for it's application, properly valved and with the spring operating in it's design envelope) an air shock is compromised and certainly does not offer any performance advantage. I have discussed some of their disadvantage, there are some others too. Yes thy are cheaper than coilovers and easier to package. Otherwise there is no good reason to run them.

I have no doubt that the performance of Billavista's suspension improved markedly and the air shocks were part of that, but I don't see this article as an unqualified endorsement of them.

Part of the issue with the inconsistent spring rate airships provide is that shock valving needs to change with spring rate, but the shock valving doesn't change as spring rate changes in an air shock. Lets say the spring rate works out at 100lb/" at your ride height with 4" of compression travel - based on the low gas pressure graph provided by Billavista there are huge changes in spring rate with a couple of inches of compression. this is why it was common in Tuff Truck to run these shocks with very low gas pressure and use hydraulic bump stops, swaybars and suck down winches to try and get around the problems having zero spring rate for most of the suspension travel caused.

Forgive me i you've already thought through all of this and have solved these problems, but I don't know anything about your build specifically, but I can say I've spent nearly 20 years working on my car. It's had three front suspension setups and five different setups in the rear. It's not easy to get this stiff right first go and it's hard to be critical of your own work - Like everyone I'd tend to focus on the positives and downplay the compromises until I put in enough seat time that I could no longer ignore the things I didn't like and I'd have another go.

This is just intended as advice based around my experience of building a link suspension for difficult terrain on my car. If you commit to air shocks, all the same advice applies, just read airshocks instead of spring and shock.

I don't run swaybars on my car. I don't need them. I put a lot of work into my suspension geometry, shock and spring placement and my car is relaxing to drive and has no quirks on road, on fast dirt, or crawling. I run Calmini Vitara rear coils all round with tender springs on the rear to keep them loaded, and King 2.5" remote reservoir smoothies, 12" travel. I run stock Sierra front bump stops all round and about 60mm of front compression and 100mm rear compression, which is pretty much the same as a stock sierra with no suspension lift. Getting the geometry right (including shock and spring placement) is very, very hard. GQ diffs will make it easier because you'll have more room between the chassis and the wheel, but their huge weight and bulk create their own problems, mostly the need to run the car very high in the front to keep the diff centre away from the chassis, which creates another set of problems to manage.

The achilles heel of the sierra chassis is the chassis width in the rear. It basically forces the shock/spring inside the chassis rail which results in very low rear roll stiffness and therefore poor behaviour in steep terrain.

It might be possible to exploit the width of the GQ axles to run the air shocks in a similar position to an 80 series land cruiser - almost tucked inside the wheel rim and below the axle centreline at the wheel but inside the chassis rail at the top. Getting the airshock inclination to work with the roll centre of the suspension rather than fight it will help to counter some of the funky spring rate issues. The old days of seeing thrown together tuff trucks with the airshocks almost touching in the centre of the car at the top is long gone - we all know a lot more about making suspension work properly now.

I refabricated the rear of my chassis to remove the taper (sierra chassis rail spacing widens from the rear of the transfer case to the back of the car) an then notched the chassis to clear my coils and shocks. This got the springs out as close the wheels as possible with greatly improves roll stiffness and tunability. It also improves the function of the shock, very important in an airshock, but as I've said the width of the GQ axles (and the small diameter of an airshock) might mean this isn't important.

I don't know how you will go packaging a 4 link in the front. Things will get very crowded between the sump, diff housing and pumpkin. The only real advantage I can see with a double triangulated 4 link is the ability to put the roll centre exactly where you want, but if the car becomes otherwise compromised because you can't package the 4 link well ( i.e the car ends up excessively tall) there might be no net gain, and the much easier packaging of a 3 link and panhard might be advantageous.

I run my shocks onto the top of my steering knuckles. Nobody does this and lots of people tell me it can't be done (which is what people like to say when they haven't seen something before) but I did this to get the bottom of the front shocks as close to the wheel as possible, again, to get the most out of the shock by getting it as close to the axle path as possible. This is important for stability.

One big overarching mantra though is this: If a modified 4WD doesn't handle well on road, it doesn't handle well off road - if a car is unstable when cornering on road it's also going to unstable on big side angles. Lateral force is the equivalent of side angle. If a car is squirrely under hard brakes on tarmac it will be spooky on steep descents. Don't fall into the "it's sick in the bush but sketchy on the road" line. It's not a distinction. I will entertain the idea that the roll centre needs to be lower for good handling on road, but that's very traction dependant. on a soft offroad tyre traction is relatively low on road so the issues caused my a high roll centre are less noticeable. (Does anyone have an opposing view to this?) It' also worth noting that the classic range rover/Discovery 1 has a high rear roll centre with no swaybars and is one of the best handling 4WD's on and off road ever built. (until people ruin them with excessive lift, and stiff "lift" springs)

There are excellent coilover spring calculators available. You won't be respringing the car multiple times. Build it, weigh and and buy springs.
Some vendors will allow you to swap springs until you get it right. Coilovers also come up for sale secondhand freqently through "comp truck" and offroad racing facebook pages.

One other comment though, which will be predictable from people who know me - make sure that after all this work what your build is actually more capable than a much smaller, lighter and more compact build. GQ diffs are huge and the resulting car won't have the line choice of a smaller car and that tends to force you into the same lines as all the "big" cars and eliminates the principle advantage of a sierra. I've gone to great pains to retain WT sierra track width and the lightest, smallest axles I can because that maximises the advantage of the suzuki platform. The exception is I've stretched the wheelbase to 100" as this is the sweet spot for my local terrain and a 35" tyre.

[Sn00dles]

Thanks for all the effort you've put into your message that's a lot of new info to think about!

I never saw the article as a glowing endorsement and am fully aware that air shocks are a compromise, it just seems like one that may be worthwhile especially seeing as I can swap it out with a coilover later if I want without needing to do much work (yes negating the cost benefit but that's ok long term). However I will look into coilovers more as if you can swap them out for no cost (with some suppliers) and there are tools available that does make them a more attractive option.

I appreciate your info on your own build as well as the future obstacles you see in my own build as they are ones I hadn't fully considered yet. I am still very much in the planning stage and the only things set in stone are GQ diffs and an SJ70 chassis so any information on suspensions setups is valuable. It is clear that I will need to know lots more about suspensions set ups as the only real research I've done is on 4 link geometry, if you have any good articles please share them!

Cheers